According to Rules 2-3-10, 2-32-16i, and 9-4-3n, an illegal blindside block involves these criteria:
1. Forceful Contact: The Colorado interpretation of "forceful" is the opponent is knocked to the ground.
2. Outside the Free Blocking Zone: The block occurs outside the free blocking zone.
3. Without Open Hands: The blocker does not use open hands when making contact.
4. The opponent does not see the blocker approaching.